Choice of Vandersteen models

I think the upgrade program is for original owners, but going through someone like John Rutan at Audioconnection might grease the wheels...
As for your original question,
It depends upon how large your room is. I own the Treo’s (non CT), and they work perfectly in my 20x18 room-even in non traditional positioning.
I would expect the Quatro’s to work as well in the same space.
The 5’s might overwhelm a room that size. Once again, Johnny would be the best person to give advice on that subject.
As far as ’holding value’ goes, once a newer model comes out the older one drops. In my opinion, I think the Quatro to be the best in terms or resale ability, and the easiest to work with in a wide range of rooms.
B

I would go with Quattro's if your just comparing them to a Model 5. But you should seriously get yourself a 5A if buying new or from a local retailer. You may be able to get a demo pair for a decent price.

The first version of the wood Quatro has a lower midrange suckout. Kind of like some older DM B&Ws but worse. 

the 5a is no longer in production, so we are talking used
they are about tge same money 5a and Q-Wood used
the mid suckout was floor bounce and was solved in collaboration with the reviewer who noted it ( and gave a glowing review ) later models have a tweak to make the room interaction less critical, but you could call factory.
As for value used later...they are pretty much tied today, easy form factor and WAF with the Q, better ( much ) lower bass in the 5a assuming you are in a room that allows for that..
as to uogrades, available to origional purchasers only, and priced that way.
long time 5a and now 7 owner with a pair of Treo coming...

speak to John Rutan at Audioconnection. 5s need HUGE room college auditorium size.